The present invention relates to a warning system for an airborne receiver in a Microwave Landing System. More particularly, it relates to a system for alerting an aircraft pilot to conditions under which the output signals produced by an airborne receiver of a microwave landing system are unsafe to utilize for aircraft guidance during approach to an airport and landing of the aircraft.
The Microwave Landing System (MLS) is a new international standard navigational aid intended to replace the fixed beam Instrument Landing System (ILS) currently in use throughout the world to provide guidance signals to an aircraft during approach and landing under low visibility conditions.
The ILS operates to provide lateral guidance signals, often referred to as localizer or azimuth guidance signals, by projecting a pair of continuous beams of a carrier frequency in the 110 MHz band in the direction of the approach path to an airport runway. The axes of the beams are skewed to the right and to the left of the runway centerline so that a receiver located thereon will detect equal strength signals from both beams while a receiver displaced laterally from the runway centerline will detect a higher strength signal from the beam having its axis nearer the receiver than from the farther beam. The beams are identified as to right or left sector coverage by modulating the carrier of one beam with a 90 Hz tone and the carrier of the other beam with a 150 Hz tone.
The ILS operates in a similar fashion to provide vertical guidance signals, often referred to as glide slope or elevation signals, except that there the beam carrier frequencies are in the 330 MHz band and the beam axes are inclined upwardly at slightly different angles from the runway surface to define a plane of equal strength signals from both beams which intersects the runway surface at a fixed glide slope angle, usually 3 degrees.
The MLS operates to provide lateral or azimuth guidance signals by projecting a fan shaped beam which is narrow in the horizontal plane and broad in the vertical plane generally along the direction of approach to the runway. Rather than being fixed in direction, as are the ILS beams, the MLS azimuth beam scans at a predetermined constant rate "TO" and "FRO" between limits of as high as .+-.60 degrees about the runway centerline. Knowing the beam scan rate, the angular displacement of a receiver from the runway centerline can be calculated by measurement of the time elapsing between detection at the receiver of signals from the "TO" beam scan and detection of signals from the next following "FRO" beam scan.
The MLS operates to provide elevation, or glide slope guidance signals in a similar manner except that there the fan shaped beam is broad in the horizontal plane and narrow in the vertical plane. The elevation beam is scanned vertically "TO" and "FRO" between limits of from +0.9 degree to as high as +30 degrees.
Apart from the manner in which guidance signals are generated, other substantial differences exist between ILS and MLS. In ILS the localizer and glide slope beams are fixed in space and are continuously transmitted on different carrier frequencies. In MLS both the azimuth and elevation beams are scanned and are transmitted on the same carrier frequency in the microwave C-band (5000 MHz). The MLS azimuth and elevation beams are time multiplexed in accordance with a standard signal format which includes provisions for transmitting much useful information from the system ground equipment to the aircraft being served, in addition to the vital proportional guidance signals.
Time multiplexing and beam scanning, complicates a warning system for the airborne MLS receiver, as compared with a warning system for an ILS receiver. In an ILS receiver a suitable warning system simply comprises a signal strength monitor for the beam r.f. carrier and a signal strength monitor for the beam modulation signal. A decrease in strength of either of the monitored signals below a tolerable amount for longer than a tolerable time causes a warning flag to appear on the pilot's course deviation indicator (CDI).
The time multiplex method of data transmission used in the MLS employs a standard signal format, later more fully described herein, in which the various functional beams, i.e. elevation and azimuth, are transmitted in a prescribed sequence. Each beam function is preceded by the transmission of a digital preamble which identifies the function to follow. In one such sequence, the time slots for beam transmissions are arranged in the following order: approach elevation; flare; approach azimuth; flare; approach elevation; back azimuth; approach elevation. The time slots for such functions are hereinafter referred to as "frames". The warning system for the MLS receiver must verify the validity of the data for each frame according to the below listed criteria. If valid data is not received for at least 45% of the frames for any function, a warning flag must be displayed to the pilot within one second of such failure.
A valid frame for any azimuth function is defined as one that satisfies all of the following requirements:
(1) Valid preamble. PA0 (2) Scanning beams symmetrical to midscan time with .+-.40 microseconds.
(3) Rate of change of indicated angle less than 2 degrees/second.
(4) Beamwidth less than 4 degrees.
(5) Scanning beam (or clearance pulse) at least 2 dB greater than out-of-coverage (OCI) pulses during acquisition and validation modes (flagged), and greater than OCI pulses in tracking mode (unflagged).
A valid frame for the elevation function satisfies requirements (1) through (3), above, and the further requirement that beamwidth must be less than 2.5 degrees.
Upon start-up, a warning flag must be displayed until signal is acquired and the validity thereof is verified for at least 45% of the frames for any function. Display of the flag must continue in the acquisition or validation mode if the amplitude of a multipath signal is within 2 dB of scanning beam amplitude. A multipath signal is a signal produced by reflection of the beam by objects on the ground or by other aircraft. When validation occurs the receiver enters the tracking mode. In the tracking mode, the multipath amplitude is allowed to be 2 dB greater than the scanning beam amplitude without generating a warning.
An MLS receiver includes the necessary r.f., i.f. and detector circuits to detect the digital data and guidance information transmitted by the MLS ground facility. The receiver further includes A/D converters for converting the analog video signals detected from the directional beams to digital form, random access memories (RAM) for storing the digitized video signals and a microprocessor for processing the received data. In prototype units, as well as in the present invention, reception of a preamble to a beam function transmission, i.e. an elevation or azimuth scan, caused the microprocessor to interrupt data processing, test the preamble code for validity and store the digitized video signals generated by the beam scan according to the beam function and time of reception. After the end of interrupt, the microprocessor determines the times of reception of the peak video signals resulting from the TO and the FRO beam scans and calculates from such times the angular displacement of the receiver from the beam mid-scan position. The resultant data is then tested for validity against the above mentioned criteria. After reception of the first valid scan signals for a particular function, tracking gates are established around the time of reception of the TO and FRO peak video scan signals. The peak amplitudes of video signals for subsequent scans of the same particular function are compared for signals inside the tracking gates with signals outside the tracking gates to determine whether multipath signals are present and if so, whether such multipath signals are at an innocuous level.
In a prototype warning system the various validity checks and multipath amplitude comparisons were integrated into a system warning by a pair of counters one of which, the frame counter, was incremented each time a frame met all of the following criteria: valid preamble identification; tracking gates symmetrical about mid-scan; beam width within limits. The frame counter was decremented each time the rate of change of the indicated receiver displacement angle exceeded 2 degrees/second. The frame counter was automatically decremented at one half the frame data rate.
The second counter of the pair termed a confidence counter, was incremented each time the amplitude of the signal within the tracking gate exceeded the amplitude of any signals outside the tracking gate. The confidence counter was decremented each time the amplitude of any out-of-gate signal exceeded the amplitude of the in-gate signal.
A system warning flag was displayed until the frame counter accumulated a count corresponding to reception of 50% valid data frames and until the confidence counter reached a count corresponding to reception of in-gate signals for one second. Whenever the accumulated counts of both counters exceeded such thresholds, the warning flag was removed. After removal of the warning flag the confidence counter continued to accumulate count for about 20 seconds. The additional accumulation of count in the confidence counter allowed strong multipath signals to persist for as long as 20 seconds after acquisition of a direct path signal without generating a warning.
The prototype warning system lacked the desirable quality of hysteresis. The frame and confidence counters incremented and decremented count in a linear fashion whether accumulating count when in a flagged condition, towards the threshold at which the system warning flag is removed or decrementing count, when in an unflagged condition, towards that same threshold at which the system warning flag will be displayed. Such lack of hysteresis led to instabilities in the warning system when the quality of the received signals was only marginally within the validity acceptance criteria.